The objective optics used in digital cameras are typically designed so as to minimize the optical point spread function (PSF) and maximize the modulation transfer function (MTF), subject to the limitations of size, cost, aperture size, and other factors imposed by the camera manufacturer. The PSF of the resulting optical system may still vary from the ideal due to focal variations and aberrations. A number of methods are known in the art for measuring and compensating for such PSF deviations by digital image processing. For example, U.S. Pat. No. 6,154,574, whose disclosure is incorporated herein by reference, describes a method for digitally focusing an out-of-focus image in an image processing system.
It is also possible to add a special-purpose blur to an image so as to create invariance to certain optical aberrations. Signal processing is then used to remove the blur. In one technique of this sort, known as “Wavefront Coding,” a special aspheric optical element issued to create the blur in the image. This optical element may be a separate stand-alone element, or it may be integrated into one or more of the lenses in the optical system. Optical designs and methods of image processing based on Wavefront Coding of this sort are described, for example, in U.S. Pat. No. 5,748,371 and in U.S. Patent Application Publications US 2002/0118457, US 2003/0057353 and US 2003/0169944, whose disclosures are incorporated herein by reference.
As another example, U.S. Pat. No. 6,927,922, whose disclosure is incorporated herein by reference, describes a system for imaging with a circularly-symmetric multifocal aspheric lens. The multifocal aspheric lens provides a blurred image, which is processed using inverse filtering, matrix convolution, or maximum entropy to obtain an extended depth of field.
U.S. Patent Application Publication US 2006/0256226, whose disclosure is incorporated herein by reference, describes an electronic imaging camera comprising an image sensing array and objective optics. The optics image an object onto the array with a point spread function (PSF) of extent substantially greater than the pitch of the sensing array even at optimal focus of the optics. As a result, the images captured by the array itself are blurred. An image processor, which may be integrated in the camera, applies a deblurring function—typically in the form of a deconvolution filter—to the signal output by the array in order to generate an output image with reduced blur. This sort of processing may similarly be applied to mosaic images, i.e., to images produced by cameras that use a single solid-state image sensor with a multi-colored mosaic filter overlay, as described, for example, in PCT International Publication WO 2007/054931, whose disclosure is incorporated herein by reference.
The above-mentioned US 2006/0256226 describes an iterative method of optical design, which takes into account the digital deblurring capabilities of the camera. This sort of method is described further in U.S. Patent Application Publication US 2007/0236573, whose disclosure is also incorporated herein by reference.
PCT International Publication WO 2007/054938, whose disclosure is incorporated herein by reference, describes an optical imaging assembly that may be used in a digital camera to generate a distorted image, which is then corrected by a deconvolution engine. The optical imaging assembly is configured to produce a high defocus aberration coefficient, which causes the modulation transfer function (MTF) of the assembly to have generally equal low values for all objects in a large field, typically from infinity to approximately 10 cm from the assembly. The deconvolution engine may be configured to improve the MTF at the different object distances and thus to produce images that are substantially free of aberrations for all objects within the field.